Generation of estimation for earth&#39;s gravity for use in measuring motion of an object

ABSTRACT

Disclosed is a method for generating an estimation of earth&#39;s gravity. The method includes: obtaining one or more acceleration data values and one or more orientation data values over a period of time; generating magnitude of orientation change from the orientation data values; determining a stability value based on the acceleration data values and the magnitude of orientation change; comparing the determined stability value to a threshold value; and generating an estimation of earth&#39;s gravity over the period of time on the basis of the acceleration data values if the comparison indicates that the determined stability value is below the threshold value. Also disclosed is an apparatus implementing the method.

TECHNICAL FIELD

The invention concerns in general the technical field of measurementsystems. Especially the invention concerns determination of earth'sgravity.

BACKGROUND

Development of sensor techniques as well as utilization of those invarying contexts has enabled more sophisticated ways to obtain andgenerate information of objects. An important application area ofsensors is monitoring of motion of an object, such as a mobile device.The monitoring provides information by means of which it is possible toderive multiple types of information, such as information on motion of adevice.

The determination of motion information on devices is traditionallyperformed by utilizing positioning information from e.g. the GlobalPositioning System (GPS). The idea behind GPS-based systems is that byanalyzing the change of position of the device some conclusions onmotion can be achieved. However, GPS-based systems have many drawbacks.Firstly, the GPS receivers suffer from high power consumption, which isa problem with mobile devices. Secondly, the mobile device shall “see” anumber of satellites in order to receive enough information fordetermining its position. This is not possible indoor and thus thedetermination of motion fails. Further, current GPS-based solutionsprovide only modest accuracy when a fine-grained distinction of e.g.transportation modes based on motion information is required.

Due to drawbacks of GPS-based systems some other solutions based onsensors for detecting motion of a device are developed. The solutionsare typically based on a utilization of accelerometer sensors. By meansof analyzing the sensor information conclusion on the motion may bederived at least to some accuracy. However, the problem with theaccelerometer sensors is that they are affected by the earth's gravityand thus the measurement information comprises a source of error. Insome state of the art implementations the earth's gravity component istaken into account opportunistically, when the device is determined tobe stationary or use of the mean of the accelerometer measurement over awindow. The former depends on frequent stationary periods, whereas thelatter has poor accuracy with continuous acceleration. An alternativelytechnique is to use the L2 norm of accelerometer measurement. Even inthat case the acceleration values of L2 norm are dominated by earth'sgravity and thus effectively masking horizontal acceleration, which isimportant parameter e.g. in motorized transportation.

In some solutions a combination of the GPS-based system and acceleratorsensor based system is developed, but the mentioned drawbacks remain, asthe solutions are distinct to each other.

Worthwhile to mention is that in addition to earth's gravity also otheracceleration related factors, such as a dynamic orientation of a devicewith respect to an entity in which the device is travelling, causeschallenges. In other words, any so called external force in addition toeffects originating from the motion of the device itself shall be takeninto account in order to determine accurate computational results asregards to determination of the gravity component.

Thus, there is need to develop a solution in order to improve theaccuracy of the measurement systems of prior art. Especially, it wouldbe advantageous to sophistically determine a gravity component so thatit takes into account a great amount of forces affecting the apparatus,i.e. the sensors, and utilize the estimated gravity information in anyfurther need, such as in motion recognition of an apparatus.

SUMMARY

An objective of the invention is to present a method, an apparatus and acomputer program product for determining an estimation of earth'sgravity. Another objective of the invention is that the method, theapparatus and the computer program product for determining an estimationof earth's gravity takes into account changes in orientation of anobject.

The objects of the invention are reached by a method, an apparatus and acomputer program product as defined by the respective independentclaims.

According to a first aspect, a method for generating an estimation ofearth's gravity is provided wherein the method comprises: obtaining oneor more acceleration data values and one or more orientation data valuesover a period of time; generating magnitude of orientation change fromthe orientation data values, determining a stability value based on theacceleration data values and the magnitude of orientation changegenerated from the orientation data values, the stability valueindicating the stability during the period of time; comparing thedetermined stability value to a threshold value; and generating anestimation of earth's gravity over the period of time on the basis ofthe acceleration data values if the comparison indicates that thedetermined stability value is below the threshold value.

The stability value may be determined by summing up the following:standard deviation of the acceleration data values during the period oftime, difference in mean of the acceleration data values during theperiod of time, magnitude of orientation change from the orientationdata values during the period of time.

The estimation of earth's gravity may be generated by determining a meanof the acceleration data values during the period of time.

The estimation of earth's gravity between those two periods of time,whose determined stability values are below corresponding thresholdvalues, may be generated by interpolating the acceleration valuesbetween the two periods of time.

The method may further comprise: comparing the estimation of the earth'sgravity generated by interpolation with at least one estimation ofearth's gravity generated through a comparison of the stability value toa threshold value in order to detect deviating estimations.

The threshold value may be adjusted in response to a determination thatthe stability value is below the threshold value over the period oftime. The adjustment of the threshold value may be performed byincreasing the threshold value by a predetermined factor. Alternativelyor in addition, the adjustment of the threshold value may be performedby setting a previous stability value being below the threshold value toa new threshold value.

According to a second aspect, an apparatus for generating an estimationof earth's gravity, the apparatus comprising at least one processor; andat least one memory including computer program code is provided whereinthe processor being configured to cause the apparatus at least toperform: obtain one or more acceleration data values and one or moreorientation data values over a period of time; generate magnitude oforientation change from the orientation data values; determine astability value for the apparatus based on the acceleration data valuesand the magnitude of orientation change generated from the orientationdata values, the stability value indicating the stability during theperiod of time; compare the determined stability value to a thresholdvalue; generate an estimation of earth's gravity over the period of timeon the basis of the acceleration data values if the comparison indicatesthat the determined stability value of the apparatus is below thethreshold value.

The apparatus may be configured to determine the stability value bysumming up the following: standard deviation of the acceleration datavalues during the period of time, difference in mean of the accelerationdata values during the period of time, magnitude of orientation changefrom the orientation data values during the period of time.

The apparatus may be configured to generate the estimation of earth'sgravity by determining a mean of the acceleration data values during theperiod of time.

The apparatus may further be configured to generate the estimation ofearth's gravity between those two periods of time, whose determinedstability values are below corresponding threshold values, byinterpolating the acceleration values between the two periods of time.

The apparatus may further be configured to: compare the estimation ofthe earth's gravity generated by interpolation with at least oneestimation of earth's gravity generated through a comparison of thestability value to a threshold value in order to detect deviatingestimations.

The apparatus may be configured to adjust the threshold value inresponse to a determination that the stability value is below thethreshold value over the period of time. Further, the apparatus may beconfigured to perform the adjustment of the threshold value byincreasing the threshold value by a predetermined factor. Alternativelyor in addition, the apparatus may be configured to perform theadjustment of the threshold value by setting a previous stability valuebeing below the threshold value to a new threshold value.

The apparatus may comprise at least one acceleration sensor and at leastone gyroscope.

According to a third aspect, a computer program product is providedwherein the computer program product comprises portions of computerprogram code configured to perform at least part of the method asdescribed above when at least some portion of the computer program codeis executed in a computing device.

The exemplary embodiments of the invention presented in this patentapplication are not to be interpreted to pose limitations to theapplicability of the appended claims. The verb “to comprise” is used inthis patent application as an open limitation that does not exclude theexistence of also un-recited features. The features recited in dependingclaims are mutually freely combinable unless otherwise explicitlystated.

The novel features which are considered as characteristic of theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF FIGURES

The embodiments of the invention are illustrated by way of example, andnot by way of limitation, in the figures of the accompanying drawings.

FIG. 1 illustrates schematically an example of an apparatus according tothe invention.

FIG. 2 illustrates schematically an example of a method according to theinvention.

FIG. 3 illustrates schematically a method step in more detail.

DESCRIPTION OF SOME EMBODIMENTS

FIG. 1 illustrates schematically, as an example, an apparatus 100according to the invention. The apparatus 100 comprises one or moreprocessors 110, one or more memories 120 being volatile or non-volatilefor storing portions of computer program code 121 a-121 n. The apparatusmay also comprise first and second sensors, wherein the first sensor isconfigured to measure acceleration 130 and the second sensor 140 isconfigured to measure orientation. The acceleration sensor 130, i.e. thefirst sensor, may be any applicable type of acceleration sensor suitablefor measuring the acceleration in three spatial dimensions (3D).Alternatively, the measurement of acceleration may be arranged with morethan one accelerators of 2D type. The orientation sensor 140, i.e. thesecond sensor, may e.g. be a gyroscope arranged in the apparatus 100. InFIG. 1 it is disclosed that the sensors are arranged in the apparatus,but it may also be arranged that the sensors are not physical parts ofthe apparatus, but may be communicatively coupled to the apparatus andcoupled to an object, whose motion is configured to be monitored. Thecommunication between the processor 110, the memory 120 and the sensors130, 140 may internally be arranged e.g. through a data bus arranged inthe apparatus 100.

The processor 110 of the apparatus 100 is configured to perform a methodaccording to the present invention as will be described. Moreover, theprocessor 110 may be configured to control the operation of theapparatus 100. The controlling may be achieved by arranging theprocessor 110 to execute at least some portion of computer program code121 a-121 n stored in the memory 120 causing the control operations bythe processor 110. The processor 110 is thus arranged to access to thememory 120 and retrieve and store any information therefrom and thereto.Moreover, the processor 110 may be configured to control measurementoperations of the sensors 130, 140 and obtain information from thesensors. The processor 110 may also be configured to control a storingof generated information. For sake of clarity, the processor hereinrefers to any unit suitable for processing information and control theoperation of the apparatus, among other tasks. The mentioned operationsmay e.g. be implemented with a microcontroller solution with embeddedsoftware.

Depending on the type of the apparatus the apparatus may furthercomprise user interface for interacting with a user and a communicationinterface for communicating with any external entity. Naturally, anyknown way of providing necessary energy within the apparatus is applied,such as arranging a battery in the apparatus or providing the power fromexternal source of energy. Further, an internal clock for providing aclock signal in the apparatus may be arranged in the apparatus 100, orit can be brought from an external source of clock signal.

Some non-limiting examples of an apparatus 100 as described may bemobile communication device, tablet computer, laptop computer,wrist-computer, a specific circuit connectable to other apparatus,devices or systems, and so on.

Next an example of a method according to the invention is described byreferring to FIG. 2. In a first phase an apparatus as described isconfigured to obtain measurement values from sensors 210. Themeasurement values comprises at least acceleration data values from oneor more accelerators and orientation data values from one or moregyroscopes. The measurement values may be obtained continuously or overcertain period of time. However, an analysis for the obtainedmeasurement data is performed to data values over a predetermined periodof time. The obtained orientation data values are configured to bemanipulated so that magnitude of orientation change is generated 220.The magnitude of orientation change herein refers to information whichtakes into account overall change in apparatus orientation on the basisof measurement information from gyroscope. The manipulation isadvantageously performed to measurement data values obtained over thepredetermined period of time at a time. The magnitude of orientationchange over the predetermined period of time may e.g. be determined byfirst calculating a mean of the data values for orientation change andthen taking a L2 norm from the mean value in order to determine themagnitude of orientation change for the period of time.

In next phase a stability value is determined for the apparatus 230,whose motion is evaluated. By means on the stability value, whichindicates the stability of the apparatus during the period of time inquestion, it is possible to determine a type of motion of the apparatus100 in question is experiencing during the predetermined period of time.According to the present invention the stability value is derived fromthe obtained acceleration data values and magnitude of orientationchange derived from the orientation data values. According to anembodiment of the invention, as depicted in FIG. 3, the stability valuemay be determined 230 by summing up at least the following: standarddeviation of the acceleration data values during the period of time 310,difference in mean of the acceleration data values during the period oftime 320, magnitude of orientation change from the orientation datavalues during the period of time 330. The one or more processors withinthe apparatus are advantageously configured to perform the determinationof the stability value as described.

In some implementations of the invention the mentioned data values usedin the determination of the stability value are configured to beweighted. An example of the weighting may be that the standard deviationof the acceleration data values receives a weight of 20%, the differencein mean of the acceleration data values receives a weight of 60% and themagnitude of orientation change from the orientation data valuesreceives a weight of 20%. The weights may be adjusted e.g. according toa utilization area of the invention, for example. The weighting enablesnormalization of the mentioned values into the same scale.

The determination of the stability value 230 as described isadvantageous due to a fact that it enables distinction between relevantacceleration information and acceleration noise information. In otherwords, the mentioned determination provide a tool for identifying twomain sources of gravity estimation errors, namely turning periods whenthe accelerometer is affected by near constant centripetal force andsustained acceleration or braking periods when the accelerometer isaffected by a constant horizontal acceleration. This is achieved bytaking into account the magnitude of orientation change from theorientation data values obtained from a gyroscope in addition toderivatives from the acceleration data values.

Reverting back to FIG. 2 the determined stability value is configured tobe compared to a threshold value 240. The threshold value may beexperimentally determined and fixed. In some implementations thethreshold value be determined based on the application area of theapparatus, e.g. if it is known that the apparatus is used in cars andmotorcycles the threshold may be determined based on this backgroundinformation. In some preferred implementations of the invention thethreshold value is adjusted during the estimation of the gravity.According to a first embodiment the adjustment may be performed bymodifying the existing threshold with a predetermined factor, such aswith a percentage value. The adjustment may also be performed so thatthe predetermined factor is determined during the running time of themethod, i.e. estimation of the earth's gravity, according to theinvention. For example, it may be arranged so that the motion ismonitored and the predetermined factor is defined according to monitoredmotion when at least one parameters relating to the motion is evaluated.Alternatively or in addition, the adjustment may be arranged so that thethreshold value is initially set to some value, e.g. by giving thedetermined stability value determined from data values from the firstperiod of time. As the next stability value is determined for the nextperiod of time, the new stability value is compared to previousstability value, which is set as the threshold value. The adjustment maybe continued as long as the apparatus is in motion, for example, or itis also possible to set some further criteria for the adjustment. Byadjusting the threshold adaptively it is possible to meet at least someinertial noise factors originating e.g. from vehicle type, road andtraffic conditions and user behavior. Thus, by adjusting the thresholdit is possible to generate a high quality gravity estimate, as is andwill be discussed herein.

The comparison step 340 is configured to be implemented so that if thedetermined stability value is above the threshold value it is concludedthat the apparatus is not in such a motion, like in constant motion orstationary that estimation on gravity can be generated and the processis continued (see the arrow back to step 210). On the contrary, if thecomparison indicates that the apparatus is in constant motion orstationary, or at least within limits thereto on the basis of thecomparison, an estimation of earth's gravity may be generated 250.

The generation of the estimation of earth's gravity affecting theapparatus over the period of time is configured to be determined 250 onthe basis of the acceleration data values if the comparison indicatesthat the determined stability value of the apparatus is below thethreshold value. According to an embodiment of the invention theestimation of earth's gravity is generated by determining a mean of theacceleration data values during the same period of time for which thestability value is determined 230.

According to an embodiment of the invention the method comprises a stepin which multiple comparison results are taken into account in thegeneration of the estimation for the earth's gravity. Namely, as alreadymentioned the determination of the stability value may be a continuousprocess and it is configured to be performed sequentially for apredetermined time window, i.e. period of time, such as 1 s. Thecomparison of the plurality of stability values determined for multipleperiods of time may indicate that only some of the periods of timecomprise such data values that are suitable for gravity estimation. Inother words, during a total time the method is performed there may bemultiple stability values, and thus periods of time, which fulfill thecriteria set for the comparison. In such a situation it is possible topick up the acceleration values, which belong to such time frames inwhich the stability value fulfills the criteria, and thus determining anestimate of gravity by e.g. taking a mean from the picked upacceleration values. Moreover, it is possible to determine an estimationof gravity also for such periods of time, whose stability value did notmeet the criteria set for the comparison. Namely, the gravity estimatefor such periods of time may be derived by interpolating an estimationof the gravity for such periods of time on the basis of estimatedgravity values originating from such periods of time wherein thecriteria set for the comparison, i.e. the stability value is below thethreshold value, is met. In other words, the estimations of earth'sgravity obtained from previous successful period of time and from theconsequent successful period of time may, according to an embodiment ofthe invention, be used in the interpolation. In such a manner it ispossible to generate the estimation of earth's gravity for the wholemonitored time.

The interpolated estimation of gravity also provides a tool fordetecting deviating estimations of gravity values from the whole set ofestimations. Namely, as the interpolated estimation providesmathematically a value, whose reliability is higher than a reliabilityof one estimation derived through the method as depicted in FIG. 2, theinterpolated estimation of earth's gravity may be used for detectingdeviating gravity estimation values. For example, by simply comparingthe interpolated value to each of the generated estimation values it ispossible to improve the result e.g. by automatically ignore suchestimation which deviate over a predetermined limit from theinterpolated value. The advantage in detecting the deviating estimationsis that in such a manner it is possible to remove those data valueswhich originate from e.g. centripetal force the apparatus isexperiencing.

The interpolation may e.g. be implemented so that starting fromgyroscope values a new interpolation value is derived with a rotationmatrix window by window. The term “window” refers herein a period oftime. The interpolation value is limited with two heuristics: a) thechange in gravitation values caused by the rotation cannot be largerthan the real change in acceleration values during the window, and b)the new interpolation value is not allowed to increase a distance toprevious interpolated value when compared to accelerator values. Byusing these two limitations it is possible to filter data valuesobtained from the gyroscope. The quality of interpolated gravityestimations may e.g. be evaluated by mutually comparing the interpolatedvalues, when the interpolation is initiated from gravity estimationsdefined to both ends of the window, which is interpolated. Theinterpolation in this manner enables utilization of basic sensors and ittakes into account a noisy/dynamic environment in which the gravityestimates are determined.

It is worthwhile to mention that the period of time, i.e. time window,over which the stability value is determined may be adjusted for a need.For example, the time window may be adjusted according to an expectedaccelerometer pattern so that if it is expected that there will be sharpturns and maneuvers in the monitored motion, a shorter time window maybe set, and if long moving modalities are expected, a longer time windowmay be set. Additionally, the sampling rate of data values obtained fromthe sensors may be adjusted, which provide more or less data values forthe data window. This may be also arranged in the similar manner as theadjustment of time window.

As already referred the generated gravity estimate may be utilized fordetermination of motion information of the apparatus. More specifically,it enables a more sophisticated estimation of linear acceleration, andthus e.g. improved determination of transport mode. The generatedgravity estimate may be used for eliminating acceleration componentcaused by earth's gravity from the data values from accelerometer andthen, by projecting the gravity eliminated accelerometer measurementsonto horizontal plane to obtain an estimate of the overall linearacceleration. The linear acceleration, in turn, is an important enablerfor many applications of mobile and wearable activity recognition as itallows separating movement along horizontal and vertical planes andhelps to overcome sensitivity to apparatus orientation. Thus, as thegravity estimation is of high quality it is possible to derive moresophisticated linear acceleration values, which may be used for multiplepurposes, such as for determination of transportation mode. In additionto the determination of the transportation mode the method and apparatusas described may be utilized, but is not limited to, in evaluation ofone's driving style, evaluation of fuel consumption, inertialnavigation, evaluating of road condition as well as evaluation oftraffic situation in general. Broadly speaking the generation of theestimation for earth's gravity may be applied in any evaluation ofmotion of an object into which the mentioned sensors are coupled to andthe measurement data is obtained.

The present invention introduces a novel and inventive method forestimating the earth's gravity by taking into account the accelerationdata and orientation data. The advantage of taking the orientation datainto account in the manner as described is that impact of anycentripetal force may be mitigated in the estimation, among otherthings. This, in turn, provides improved estimations and an accuracy ofany device or system utilizing the estimated earth's gravity may beimproved in any application area. Even if it is herein mainly describedthat the invention relates to an estimation of earth's gravity affectingto an apparatus. Being more specific, the estimation is determined foran object into which the sensors are coupled to. In some implementationsof the invention the sensors reside within the apparatus, but this isnot a prerequisite of the invention as such.

Features described in the preceding description may be used incombinations other than the combinations explicitly described. Althoughfunctions have been described with reference to certain features, thosefunctions may be performable by other features whether described or not.Although features have been described with reference to certainembodiments, those features may also be present in other embodimentswhether described or not.

The invention claimed is:
 1. A method for generating estimations ofearth's gravity for sequential periods of time for use in measuringmotion of an object, the method comprising for each period of time:obtaining from an acceleration sensor one or more acceleration datavalues, obtaining from a gyroscope one or more orientation data valuesover a period of time in question, generating in a processor magnitudeof orientation change from the orientation data values, determining in aprocessor a stability value based on the acceleration data values andthe magnitude of orientation change generated from the orientation datavalues, the stability value indicating the stability during the periodof time in question, comparing in a processor the determined stabilityvalue to a threshold value wherein the threshold value is adjusted forthe period of time in question, generating in a processor an estimationof earth's gravity over the period of time in question on the basis ofthe acceleration data values if the comparison indicates that thedetermined stability value is below the threshold value, therebymitigating at least an impact of any present centripetal force; whereinthe estimation of the earth's gravity relates to an object to which theacceleration sensor and the gyroscope are coupled.
 2. The method ofclaim 1, wherein the stability value is determined by summing up thefollowing: standard deviation of the acceleration data values during theperiod of time in question, difference in mean of the acceleration datavalues during the period of time in question, magnitude of orientationchange determined from the orientation data values during the period oftime in question.
 3. A non-transitory computer-readable medium on whichis stored a program which, when executed by a computer, causes thecomputer to perform the method of claim
 2. 4. The method of claim 1,wherein the estimation of earth's gravity is generated by determining amean of the acceleration data values during the period of time inquestion.
 5. A non-transitory computer-readable medium on which isstored a program which, when executed by a computer, causes the computerto perform the method of claim
 4. 6. The method of claim 1, wherein theestimation of earth's gravity between two periods of time, whosedetermined stability values are below corresponding threshold values, isgenerated by interpolating the acceleration values between the twoperiods of time.
 7. The method of claim 6, wherein the method furthercomprising: comparing the estimation of the earth's gravity generated byinterpolation with at least one estimation of earth's gravity generatedthrough a comparison of the stability value to a threshold value inorder to detect deviating estimations.
 8. The method of claim 1, whereinthe threshold value is adjusted in response to a determination that thestability value is below the threshold value over the period of time inquestion.
 9. The method of claim 8, wherein adjustment of the thresholdvalue is performed by increasing the threshold value by a predeterminedfactor.
 10. The method of claim 8, wherein the adjustment of thethreshold value is performed by setting a previous stability value beingbelow the threshold value to a new threshold value.
 11. A non-transitorycomputer-readable medium on which is stored a program which, whenexecuted by a computer, causes the computer to perform the method ofclaim
 1. 12. An apparatus for generating estimations of earth's gravityfor sequential periods of time for use in measuring motion of an object,the apparatus comprising at least one processor; at least oneacceleration sensor; at least one gyroscope; and at least one memoryincluding computer program code characterized in that the processorbeing configured to cause the apparatus at least to perform for eachperiod of time: obtain one or more acceleration data values from the atleast one acceleration sensor and obtain one or more orientation datavalues from the at least one gyroscope over a period of time inquestion, generate magnitude of orientation change from the orientationdata values, determine a stability value for the apparatus based on theacceleration data values and the magnitude of orientation changegenerated from the orientation data values, the stability valueindicating the stability during the period of time in question, comparethe determined stability value to a threshold value wherein thethreshold value is adjusted for the period of time in question, generatean estimation of earth's gravity over the period of time in question onthe basis of the acceleration data values if the comparison indicatesthat the determined stability value of the apparatus is below thethreshold value, thereby mitigating at least an impact of any presentcentripetal force; wherein the estimation of the earth's gravity relatesto an object to which the acceleration sensor and the gyroscope arecoupled.
 13. The apparatus of claim 12, wherein the apparatus isconfigured to determine the stability value by summing up the following:standard deviation of the acceleration data values during the period oftime in question, difference in mean of the acceleration data valuesduring the period of time in question, magnitude of orientation changedetermined from the orientation data values during the period of time inquestion.
 14. The apparatus of claim 12, wherein the apparatus isconfigured to generate the estimation of earth's gravity by determininga mean of the acceleration data values during the period of time inquestion.
 15. The apparatus of claim 12, wherein the apparatus isconfigured to generate the estimation of earth's gravity between twoperiods of time, whose determined stability values are belowcorresponding threshold values, by interpolating the acceleration valuesbetween the two periods of time.
 16. The apparatus of claim 15, whereinthe apparatus is further configured to: compare the estimation of theearth's gravity generated by interpolation with at least one estimationof earth's gravity generated through a comparison of the stability valueto a threshold value in order to detect deviating estimations.
 17. Theapparatus of claim 12, wherein the apparatus is configured to adjust thethreshold value in response to a determination that the stability valueis below the threshold value over the period of time in question. 18.The apparatus of claim 17, wherein the apparatus is configured toperform the adjustment of the threshold value by increasing thethreshold value by a predetermined factor.
 19. The apparatus of claim17, wherein the apparatus is configured to perform the adjustment of thethreshold value by setting a previous stability value being below thethreshold value to a new threshold value.